Vitamin C (ascorbic acid) has proven to possess beneficial properties when applied in cosmetic products for skin care. For instance, vitamin C stimulates collagen synthesis in the skin, thereby producing a reduction of wrinkles, it has a skin-whitening effect by interfering in the formation of melanin and it can retard skin damage caused by exposure of the skin to UV light.
Since most cosmetic products are water-based, the main focus has been on the development of aqueous formulations of this vitamin. However, the poor stability of vitamin C in aqueous formulations has been identified as a major problem in this regard.
Attempts to stabilize vitamin C are well documented. Most efforts to stabilize vitamin C compositions have been directed to minimize or prevent aerobic degradation. This can be done by adding suitable stabilizing agents, such as anti-oxidants (to prevent oxidation) and/or chelators (to complex metal ions catalyzing oxidation) (Hajratwala (1985), Rev. Sc. Pharm. of March 15). Furthermore, the pH of a vitamin C composition is found to play a role: a pH below about 3.5 has been shown to improve vitamin C stability (WO 90/12572). Additionally water activities below 0.85 have been found to stabilize vitamin C (EP 755 674).
The detrimental influence of oxygen may be circumvented by applying substantially oxygen-free conditions for preparation and storage of vitamin C compositions. Although degradation of vitamin C under anaerobic conditions is much less investigated, it has been described that this type of degradation may result in the formation of furfural and carbon dioxide (Counsel and Hornig (1981), In: Vitamin C, pp 123-137, Applied Science Publishers). Fructose and fructose derivatives appeared to stimulate this furfural/carbon dioxide production under said conditions (ibid.).
During the development by the Applicant of concentrated vitamin C compositions suitable for use in a multichamber dispensing system and for packing under substantially oxygen-free conditions, it was found that extensive gas formation occurred during preparation as well as storage of vitamin C compositions.
Of course, gas production in a closed container is an undesirable situation. Gas formation is especially unwanted in those cases that vitamin C compositions are packed in airtight containers to minimize oxidative deterioration. For instance, gas formation then will result in pressure in the compartment containing the vitamin C concentrate. In the case that a vitamin C concentrate is packed in a cartridge connected to an airless pump dispensing system, gas formation may result in inaccurate dosing of the vitamin C concentrate. More importantly, if relatively large amount of gas are formed, the plunger will be pushed out of the cartridge and the vitamin C concentrate will be released in the outer compartment of the dispenser or in the environment.
In very concentrated vitamin C formulations, the aqueous phase will be saturated with dissolved vitamin C and additionally contain a significant portion of vitamin C crystals. To guarantee an even dosage from a multicompartment dispenser, it is essential that these crystals are homogeneously suspended in the aqueous phase. Therefore a stable vitamin C concentrate should not produce any gas and at the same time any vitamin C crystals present should not precipitate.
The present invention provides several solutions to accomplish this. Furthermore, the present invention advantageously uses the anaerobic condition of the stable vitamin C concentrate to protect other active ingredients prone to oxidation against detrimental effects of oxygen.